Topic 1.3 Membrane Structure Flashcards
What is the Fluid Mosaic Model
- Lipids (phospholipids, cholesterol)
- Proteins (extrinsic/peripheral and intrinsic/integral)
- Carbohydrates (glycolipids, glycoproteins)
What is the structure of a phospholipid
- Polar phosphate end is hydrophilic,
- Hydrocarbon ends with 2 fatty acid tails is hydrophobic
- Hence, the phospholipid is amphipathic
- Can form monolayers, liposomes, micelles, bilayers
Factors affecting the Fluidity of Membrane
- Saturation
- Length of fatty acid tails
- Temperature
- Cholesterol
How does Saturation of fatty acid tails affect fluidity of membrane
More unsaturated = More double bonds = more kinks = prevent tight packing = decreased number of hydrophobic interactions = greater membrane fluidity.
How does length of fatty acid tails affect the fluidity of the cell membrane
Shorter fatty acid tails = decreased number of hydrophobic interactions = more fluid
How does temperature affect the fluidity of the membrane
- Temp decreases, and membrane fluidity decreases.
2. The temperature at which membrane solidifies – phase transition temperature
How does cholesterol affect the fluidity of the membrane
At Low Temp:
- Increases fluidity at low temp by disrupting close and regular packing of phospholipids –> preventing solidification of membrane.
At High Temp:
- Decreases fluidity at high temps by restraining movement of phospholipids –> prevent membrane from disintegrating.
Structure of Cholesterol
- Amphipathic (polar OH group but other than that its non-polar)
- Head is attracted to phosphate heads of phospholipids, its tails to the tails of the phospholipids
- Synthesized in liver
- 4 fused rings
- Rarely in plant cells
- Helps membranes to curve into a concave shape.
Why is maintaining membrane fluidity important?
Membrane fluidity must be carefully controlled.
- Too fluid – unable to control the passage of substances
- Not fluid enough – movement of the cell and substance within it is restricted.
What are Extrinsic Proteins
Location:
Attached to the surface (inner-cytoplasmic, outer-extracellular),
Bonds:
- Hydrogen and ionic bonds between polar regions and polar head of phospholipids.
What are Intrinsic Proteins
Location and Structure:
- Embedded in Hydrocarbon chains of the membrane (hydrophobic on at least part of the surface)
- Hydrophobic and hydrophilic on a different parts of the surface
Bonds:
- Polar: Hydrogen and ionic bonds between polar regions and polar head of phospholipids
- Non-Polar: Weak hydrophobic interactions btw non-polar protein and hydrophobic core of Phospholipids
Types of Intrinsic Proteins
- Integral Unilateral Protein
- Does not span across the entire membrane - Integral Transmembrane Protein
- Spans across the entire membrane
Types of membrane proteins
- Receptors
- Immobilized enzymes
- Cell adhesion
- Cell to cell communication and cell recognition
- Transport
- Passive
- Carrier
- Channel
- Pump
Function of receptor membrane proteins
o Recognize and bind with specific molecules outside the cell
o Integral proteins with a binding site complementary to shape of specific ligand.
o Upon binding, receptors are activated, stimulates cellular response by signal transduction pathways
Function of immobilized enzymes
Integral proteins with active sites exposed to substrates in adjacent solution.
Examples:
- Immobilized digestive enzymes bound on microvilli
- ATPase inner membrane of mitochondria/thylakoid membrane
Function of cell to cell communication and cell recognition
- Glycoproteins serve as identification tags specifically recognized by membrane proteins of other cells.
- Short live cell-cell binding
Function of cell adhesion
o Membrane proteins of adjacent cells hook together in various kinds of junctions (e.g. tight/gap)
o Long lasting, virtually impermeable barrier to fluid and dissolved substances
Function of channel proteins
o Transmembrane with hydrophilic pore
o Passive movement of ions and hydrophilic molecules across the membrane
Function of carrier proteins
Binding of substrate = change in conformation = translocation across membrane
Function of pumps
Same as carrier proteins but hydrolyses ATP for energy to transport substance against conc gradient.
What is the sandwich model
Suggested a bilayer of phospholipids in the centre of membrane with layers of protein on either side.
- Evidence shows that the membrane has enough phospholipids to make twice as large area of membrane –> phospholipid bilayer
- Membrane form a barrier to transport some substance, suggesting layers of protein acting as the barrier
Problems with the sandwich model
- Amount and type of membrane protein varied among different cells
- Did not account that membrane proteins are mainly hydrophobic, hence the majority of the surface area should not be facing an extracellular/cytoplasmic environment
- Attempts to extract protein indicated that some occurred on the external surface but others were buried within or across the lipid bilayer and were harder to extract.
- Freeze etching studies of plasma membrane show that some protein are seen to occur buried within or across the lipid bilayers.
- Specific components “tagged’ by fluorescent dyes showed component molecules to be continually on the move within the membrane.
Why fluid mosaic model is correct
- Membrane is a phospholipid bilayer which is free to move about laterally, with a mosaic-like, random distribution of proteins in, or loosely attached to the phospholipid bilayer.
- Proved by freeze fracture (freeze sample, cut with microtome knife to split the cell and expose the membrane’s layered structure), and electron micrographs. Showed that globular proteins embedded in the membrane with the hydrophilic regions protruding.
- Found that there were both integral and peripheral proteins in the phospholipid bilayer, hence its not sandwich.
